1. Field of the Invention
This invention relates to a light emitting device and, particularly, to a light emitting device with a phosphor layer for wavelength-converting light emitted from a light emitting element.
2. Description of the Related Art
Conventionally, a light emitting device is suggested that allows the wavelength conversion of light emitted from a light emitting element by a phosphor material to obtain a desired emission color.
Such a light emitting device is, as shown in FIG. 1, composed of: a board 5; a circuit pattern 4 formed on the board 5; a light emitting element 1 mounted through the circuit pattern 4 on the board 5, the light emitting element 1 being enabled to emit ultraviolet light; a cover 2 that is formed semispherical while covering a light emitting element 1 to emit ultraviolet light, and a phosphor layer 3 that is provided on the inner surface of the cover 2. In the light emitting device, the phosphor material in phosphor layer 3 is excited by ultraviolet light emitted from the light emitting element 1 and, thereby, it radiates, e.g., white light.
Japanese patent application laid-open No. 2001-217466 discloses a light emitting device that its light emitting element is covered with sealing resin material (phosphor layer) with phosphor particles dispersed therein.
However, in the light emitting device as shown in FIG. 1, since the phosphor layer 3 is exposed, the phosphor material may be subjected to degradation due to absorbed moisture. If an air-tight housing is employed to prevent the penetration of water into the cover 2, the manufacturing cost will rise since the light emitting device becomes difficult to assemble. Even in case of dispersing phosphor particles in the sealing resin material as in Japanese patent application laid-open No. 2001-217466, the degradation of phosphor material due to moisture cannot be prevented sufficiently.
Further, since a phosphor material including metal element generally has a high specific density, it may flow by gravitation when being coated on the inner surface of cover 2. Therefore, it is difficult to make the thickness of phosphor layer 3 equal. Thus, it is difficult to generate equal fluorescence over the entire cover 2.